1. Field of the Invention
This invention relates to radio frequency transceiver design, and, more particularly, to decreasing spurious emissions.
2. Description of the Related Art
Radio frequency (RF) transmitters/receivers are used in a wide variety of applications, including wireless network interfaces, mobile telephones, and Bluetooth interfaces. RF transceivers also feature prominently in wireless audio technology directed to headphones and earphones, home audio/theater systems and speakers, portable audio/media players and automotive sound systems. Robust, high-quality audio and low-power RF capability can make it possible for consumer and automotive original equipment manufacturers (OEMs) to integrate wireless audio technology into portable audio devices and sound systems. Overall, various RF technologies lend themselves to a number of applications in the consumer world to create high-fidelity home theater environments and distribute audio in the home and other environments.
A radio communication system typically requires tuned circuits at both the transmitter and receiver. The transmitter is an electronic device that propagates an electromagnetic signal, representative of an audio signal, for example, typically with the aid of an antenna. An RF transceiver is designed to include both a transmitter and a receiver, combined to share common circuitry, many times appearing on the same piece of Integrated Circuit (IC) chip. If no circuitry is common between transmit and receive functions, the combined device is referred to as a transmitter-receiver.
RF Transceivers use RF modules for high-speed data transmission. In most systems, digital processors or processing elements (which are oftentimes software-programmable) are used to perform conversion between digital baseband signals and analog RF, and oscillators are used to generate the required periodic signals. Many RF circuits make use of a voltage-controlled oscillator (VCO), in which the oscillation frequency is controlled by a voltage input, and the oscillation frequency is controlled through an applied DC voltage.
Some systems also use injection-locking for generating the transmission oscillator signal. Injection-Locked Oscillators (ILOs) feature a local oscillator locked to the frequency and phase of an external injection signal under proper conditions, for example when the coupling between the two oscillators is strong enough, and the respective operating frequencies of the two oscillators are close enough. Alternately, ILOs are sometimes configured as Injection Locked Frequency Dividers (ILFDs), in which case the frequency of the input signal is a multiple (or fraction) of the free-running frequency of the oscillator. The locking range of an ILO is inversely proportional to the quality factor of the oscillator circuit, making ILOs sensitive to process variations in integrated circuit designs. In order for the ILO to operate properly, the tuning range of the driving circuit (for example, a VCO) should fall within the input locking range of the ILO.
RF transmitters are prone to RF leakage, causing interference by way of out of band emissions, and spurious emissions. Spurious emissions (or spurs) are signals emitted by a transmitter on a frequency or frequencies which are outside the necessary bandwidth, and typically include harmonics, leakage from the local oscillator and other systems within the transmitter, intermodulation products, and frequency conversion products. The level of permissible out of band emissions and spurious emissions in RF systems are regulated by local regulatory authorities. Therefore, most RF transmitters and/or transceivers attempt to minimize out of band emissions, as well as spurious emissions.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.